Linear polyesters such as poly(alkylene terephthalate) are generally known and commercially available where the alkylene typically has 2 to 8 carbon atoms. Linear polyesters have many valuable characteristics including strength, toughness, high gloss, and solvent resistance. Furthermore, polyesters may be fabricated into articles of manufacture by a number of well-known techniques including injection molding, roto-molding, and extrusion.
Recently, macrocyclic oligoesters were developed as precursors to polyesters. Macrocyclic oligoesters exhibit low melt viscosity, which can be advantageous in certain applications. Furthermore, certain macrocyclic oligoesters melt and polymerize at temperatures well below the melting point of the resulting polymer. Upon melting and in the presence of an appropriate catalyst, polymerization and crystallization can occur virtually isothermally.
Single-component catalysts that have been developed for use in the polymerization of macrocyclic oligoesters include various organo-metal compounds, including conventional titanate esters. The required volume ratio of a macrocyclic oligoester to a conventional titanate ester is very high, typically greater than 100:1. See, e.g., U.S. Pat. No. 5,466,744 to Evans et al. The high volume ratio requires sophisticated and costly metering and mixing equipment to properly introduce and disperse the relatively small amount of catalyst. This is of particular concern in liquid molding and extrusion applications.
Furthermore, it is desirable to develop catalytic systems of increased versatility to better control the onset and speed of polymerization without the need for costly metering and mixing equipment. For instance, it is desirable to develop catalytic systems which do not appreciably begin to catalyze a reaction until an appropriate time, and which allow the reaction to take place quickly and relatively homogeneously throughout the reaction mixture once reaction is initiated.